Rack & pinion

This idea was put aside for production reasons back in the days (easier to buy belt in 50m roll), but I can implement it again now
(so you can use belt or printed rack, both are compatible with the x-carriage once the little protrusions for the bearings are removed)

The speed limit is reached either from :
not enough motor's torque
acceleration (too high)
step rate generation (at 1/32 microstepping, a 8 bit cpu will quickly have problems following the "consumption" of step pulses)

That said, I print at 30 or 60mm/s (small or big objects), usually 40mm/s

Beside XY movement speed, the printing speed mean nothing, the important factor is the volumetric speed (how many mm3/s you can lay down).
e.g.:
7.5mm3/s
= 0.5mm width 0.25mm layers at 60mm/s
= 0.5mm width 0.30mm layers at 50mm/s
The layers being thicker you need to slow down.
(I just want to say that every time someone claim to print at xxxmm/s, with very thin layers yes it is easy)